Conventional wellhead equipment used in connection with completed oil and gas production wells include, from the bottom up, a flanged casing head having an internal casing hanger from which the casing is suspended; a flanged tubing head having an internal tubing hanger from which the well tubing string is suspended; and a “Christmas tree” consisting of multiple components, the interconnected components forming a vertical fluid flow bore which extends through the components from top to bottom. The overall wellhead assembly extending upwards from the top of the tubing head is commonly referred to as the production Christmas tree. Individual components of the Christmas tree are typically flange connected and sealed to each other top and bottom and include, upwards from the tubing head, a tubing head adapter; at least one, and commonly two, master valves (termed upper and lower master valves); a flow tee or studded cross member having side and top openings, typically studded for connecting with flow lines, one or more wing valves and additional components; a swab valve; and a cap at the upper end.
It is common in the oilfield industry to insert a string of coil tubing (also termed coiled tubing) into a completed oil or gas wellbore. The coil tubing may be used for a number of purposes such as chemical injection, servicing, or for carrying downhole sensors. The coil tubing is a continuous reeled pipe, generally of relatively small diameter, such as ¼ or ⅜ inch.
The coil tubing string is run into and out of the wellbore using coil tubing string injectors, which force the coil tubing string into the well. The coil tubing string is supported through a device known as a coil tubing hanger. The process is reversed as the coil tubing string is removed from the well.
There are generally three methods for running coil tubing into the wellbore. The first involves removing the Christmas tree cap at the top of the wellhead and then installing the coil tubing through the Christmas tree, such that it is concentric through the entire vertical section of the Christmas tree. Several problems can arise with this approach. To shut off the well in case of an emergency, the shut off valve may become inoperative because of interference with the coil tubing. The coil tubing may have to be severed by the shut off valve in order to control the well, and by doing this, the valves may be damaged. In either case, the shut off valves can become problematic or inoperative, and well control is given up. A second approach is to strap the coil tubing outside the tubing string, using an injection nipple to connect the two lines with check valves. This is a time-consuming approach to run in and install the coil tubing parallel to, and at the same time as, the tubing string. On completed wells, a rig must pull out the production tubing and reinstall with the production and coil tubing strapped together. A third technique includes adding a spool, generally in a Y-body shape, into the Christmas tree, and running the coil tubing through the Y or angled side arm into the vertical production bore. Generally this spool is connected between the two conventional valves in the Christmas tree, namely an upper master valve and a lower master valve. These valves are typically placed above the tubing head and tubing head adapter in the conventional Christmas tree of a wellhead. An example of this type of spool device is an RH-Y Capillary Spool available from Wellhead Control Products, Inc., Texas, U.S.A. This spool adds height to the wellhead, requires relocation of flowlines for existing wells, and creates significant safety issues as it becomes more difficult to work on the wellhead.